Combustion system and method for operating same

ABSTRACT

An oxygen combustion system includes a boiler to burn fuel using combustion gas composed of oxygen-rich gas and circulating flue gas, a dust remover disposed in a flue through which flue gas discharged from the boiler flows, a second flue leading the combustion gas to the boiler, the combustion gas being made by mixing the circulating flue gas extracted downstream of the dust remover with the oxygen-rich gas, a combustion gas heater exchanging heat between the flue gas flowing between the boiler and dust remover and the combustion gas flowing through the second flue, and a flue gas cooler disposed between the heater and the dust remover to cool the flue gas. A control unit controls at least one of a flow rate and cooling medium temperature of the flue gas cooler such that temperature of the flue gas introduced into the dust remover will be between 90° C. and 140° C.

TECHNICAL FIELD

The present invention relates to a combustion system, and moreparticularly, to a technique for stable combustion in a boiler of acombustion apparatus.

BACKGROUND ART

To reduce global warming and other environmental burden, there is asocial demand for reduction of carbon dioxide (CO₂) emissions. CO₂ isproduced when solid fuels, such as coal, containing carbon are burned.Therefore, there is demand to reduce CO₂ emissions produced when solidfuels are burned.

Thus, Patent Literature 1 proposes to burn fuel using combustion gasmade of a mixture of oxygen-rich gas and flue gas, thereby increase CO₂concentration in combustion flue gas, and thereby recover CO₂ form thecombustion flue gas with high efficiency. Specifically, PatentLiterature 1 proposes a boiler plant based on a so-called oxygen-basedcombustion method which separates air into gas mainly composed of oxygenand gas mainly composed of nitrogen, produces combustion gas by dilutingthe gas mainly composed of oxygen (hereinafter referred to asoxygen-rich gas) with combustion flue gas, and burns pulverized coalwith the combustion gas. Incidentally, the technique described in PatentLiterature 1 uses combustion flue gas with a low oxygen concentration asa carrier gas for gas-stream conveyance of the pulverized coal, andconsequently has a problem of ignition delay depending on the type ofcoal.

To deal with this, Patent Literature 2 proposes a technique for speedingup the ignition of pulverized coal. According to the technique describedin Patent Literature 2, in a combustion boiler plant that is, instead ofan oxygen-based combustion method, adapted to burn pulverized coal byair, the pulverized coal is transported using a stream of a carrier gaswhose oxygen concentration has been reduced by mixing combustion fluegas in air, because coal with a high volatile content might ignitespontaneously during transport when transported pneumatically. However,the reduction in the oxygen concentration in the carrier gas causesignition delay of the pulverized coal in a combustion zone. Therefore,to accelerate the ignition of the pulverized coal, the oxygenconcentration is increased by supplying additional air to the combustionzone in which the pulverized coal is burned.

CITATION LIST Patent Literature

-   Patent Literature 1: JP-A-2007-147162-   Patent Literature 2: JP 4150968

SUMMARY OF INVENTION Technical Problem

Incidentally, with the technique described in Patent Literature 2, toprevent natural ignition, the oxygen concentration in the carrier gas isset according to the type of pulverized coal. On the other hand, the airadded to the combustion zone to improve ignition performance has a fixedoxygen concentration. Therefore, flow rate of the additional air isincreased relative to flow rate of the carrier gas, thereby increasingthe oxygen concentration in the combustion zone. Therefore, if the flowrate of the additional air is increased too much, it could not produce astable reducing flame which is formed by limiting the amount of oxygensupplied to fuel in the combustion zone.

A problem to be solved by the present invention is to achieve stablecombustion in a combustion system switchable to an oxygen-basedcombustion method.

Solution to Problem

To solve the above problem, the present invention provides a combustionsystem wherein: a burner adapted to burn solid fuel comprises a fuelnozzle adapted to burn solid fuel supplied in accompaniment with carriergas, a first combustion gas nozzle adapted to supply first combustiongas into the fuel nozzle, and a second combustion gas nozzle placed onan outer side of the fuel nozzle and adapted to supply second combustiongas; the combustion system is configured to be switchable between an aircombustion mode in which air is used as the carrier gas, the firstcombustion gas and the second combustion gas, and an oxygen-basedcombustion mode in which mixed gas is used as the carrier gas, the firstcombustion gas and the second combustion gas, where the mixed gas ismade by mixing oxygen-rich gas with flue gas produced when the solidfuel is burned; respective oxygen concentrations and supplied amounts ofthe carrier gas, the first combustion gas, and the second combustion gasare each configured to be adjustable; and in the oxygen-based combustionmode, the oxygen concentration in the carrier gas is adjusted to belower than an average oxygen concentration of the carrier gas, the firstcombustion gas and the second combustion gas taken together; and theoxygen concentration in the first combustion gas is adjusted to be equalto or higher than the average oxygen concentration of the carrier gas,the first combustion gas and the second combustion gas taken together.

Consequently, even if the oxygen concentration in the carrier gas is setlow to prevent spontaneous ignition of the solid fuel during transport,it is possible to improve ignition performance by controlling the oxygenconcentration in the first combustion gas supplied into the fuel nozzle.Accordingly, an amount of oxygen in a first combustion zone in the fuelnozzle is set at a level necessary to maintain a reducing flame. Then,if a deficient amount of oxygen in a second combustion zone isdetermined based on amounts of oxygen in the carrier gas and firstcombustion gas in the first combustion zone and the oxygen deficiency ismade up by controlling flow rate of the second combustion gas, the solidfuel can be burned stably.

In this case, the fuel nozzle may include a distributor adapted todivide a flow channel in the nozzle and an outlet of the firstcombustion gas nozzle may be formed at a location overlapping thedistributor when viewed in a direction perpendicular to a burner axis.Consequently, since the flow channel is partitioned by the distributor,the solid fuel, carrier gas, and first combustion gas are kept frommixing with one another. This makes it possible to prevent dispersion ofthe solid fuel due to collisions among solid fuel particles as well asdiffusion of the first combustion gas to near a central axis. As aresult, gas with a low oxygen concentration and gas with a high oxygenconcentration can be separated at a burner outlet with the formerdirected onto the central axis and the latter directed toward an outercircumference, improving ignition performance.

Also, the fuel nozzle includes a venturi and a concentrator located in aflow channel upstream of a position at which the first combustion gasnozzle is connected, the venturi being adapted to smoothly contract andexpand a sectional area of the flow channel of the fuel nozzle from anouter peripheral side while the concentrator being adapted to smoothlyexpand the sectional area of the flow channel downstream of the venturifrom inside; and a flame holder is installed at a tip of a partitionwall partitioning the fuel nozzle and the second combustion gas nozzle,the flame holder being adapted to block flow of the solid fuel andcarrier gas ejected through the fuel nozzle and flow of the firstcombustion gas ejected through the first combustion gas nozzle. Thismakes it possible to reduce pressure on the downstream side of the flameholder, causing high-temperature gas in the boiler to flow to thedownstream side of the flame holder, and thereby improve the ignitionperformance of the solid fuel.

Also, the present invention provides a combustion system comprising: anoxygen-based combustion boiler adapted to burn solid fuel by dilutingoxygen-rich gas with flue gas; a crusher adapted to crush the solidfuel; a fuel supply device adapted to supply the crushed solid fuel to aburner of the boiler in accompaniment with carrier gas; a dust collectoradapted to collect fly ashes in flue gas discharged from the boiler; anda carbon dioxide recovery unit adapted to separate and recover carbondioxide from the flue gas discharged from the dust collector; the burnerin turn comprising a fuel nozzle adapted to burn the solid fuel suppliedin accompaniment with the carrier gas, a first combustion gas nozzleadapted to supply first combustion gas into the fuel nozzle, and asecond combustion gas nozzle placed on an outer side of the fuel nozzleand adapted to supply second combustion gas, wherein the combustionsystem further comprises a carrier gas line adapted to generate carriergas from the flue gas branching off from a downstream side of the dustcollector, a combustion gas line adapted to generate combustion gas byadding oxygen-rich gas to the flue gas branching off from the downstreamside of the dust collector, and a controller adapted to set oxygenconcentration in the carrier gas and control flow rate of the carriergas according to a supplied amount and type of the solid fuel, control asupplied amount and oxygen concentration of the first combustion gasbased on the oxygen concentration and flow rate of the carrier gas and aset amount of oxygen needed for combustion of the solid fuel, andcontrol a supplied amount of the second combustion gas based on anamount of oxygen in the carrier gas, an amount of oxygen in the firstcombustion gas, and a second set amount of oxygen needed for combustionof the solid fuel.

In this case, an adding device adapted to add oxygen-rich gas to theflue gas based on the oxygen concentration in the carrier gas set by thecontroller may be installed on the carrier gas line.

On the other hand, the present invention provides an operating methodfor a combustion system which comprises an oxygen-based combustionboiler adapted to burn solid fuel by diluting oxygen-rich gas with fluegas, a crusher adapted to crush the solid fuel, a fuel supply deviceadapted to supply the crushed solid fuel to a burner of the boiler inaccompaniment with carrier gas, a dust collector adapted to collect flyashes in flue gas discharged from the boiler, and a carbon dioxiderecovery unit adapted to separate and recover carbon dioxide from theflue gas discharged from the dust collector, the burner in turncomprising a fuel nozzle adapted to burn the solid fuel supplied inaccompaniment with the carrier gas, a first combustion gas nozzleadapted to supply first combustion gas into the fuel nozzle, and asecond combustion gas nozzle placed on an outer side of the fuel nozzleand adapted to supply second combustion gas, the operating methodcomprising: generating the carrier gas by selectively adding oxygen-richgas to the flue gas branching off from the downstream side of the dustcollector; generating the first combustion gas and the second combustiongas by adding oxygen-rich gas to the flue gas branching off from thedownstream side of the dust collector; and setting oxygen concentrationin the carrier gas and controlling flow rate of the carrier gasaccording to a supplied amount and type of the solid fuel, controlling asupplied amount and oxygen concentration of the first combustion gasbased on the oxygen concentration and flow rate of the carrier gas and afirst set amount of oxygen needed for combustion of the solid fuel, andcontrolling a supplied amount of the second combustion gas based on anamount of oxygen in the carrier gas, an amount of oxygen in the firstcombustion gas, and a second set amount of oxygen needed for combustionof the solid fuel.

In this case, at the start of combustion mode operation when the fluegas is deficient, the carrier gas is generated for operation by makingup the flue gas deficiency by air, and when the flue gas increases, theflue gas is substituted for the air. Consequently, during a start-upoperation when the oxygen concentration in the carrier gas is high, thefirst combustion gas is controlled as described above to keep down theoxygen concentration, allowing the oxygen concentration in the firstcombustion zone to be maintained in a range of reductive combustion.

With the oxygen-based combustion system, since the combustion gas isproduced by diluting oxygen-rich gas with flue gas, the CO₂concentration in the combustion gas reaches, for example, 60 to 90%,flame spread speed is drawn far low in comparison with the air,resulting a delay of an ignition. Thus, preferably the oxygenconcentration in the carrier gas is set to 21% or below and the oxygenconcentrations in the first and second combustion gases are controlledto be higher than that of air, for example, 25 to 35%.

Advantageous Effects of Invention

The present invention makes it possible to achieve stable combustion inan oxygen-based combustion system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a boiler plant according to a firstembodiment.

FIG. 2 is a sectional view of a burner in FIG. 1.

FIG. 3 is a block diagram of a boiler plant according to a secondembodiment.

FIG. 4 is a sectional view of a burner according to a third embodiment.

FIG. 5 is a sectional view of a burner according to a fourth embodiment.

FIG. 6 is a sectional view of a burner according to a fifth embodiment.

FIG. 7 is a sectional view of a burner according to a sixth embodiment.

DESCRIPTION OF EMBODIMENTS

The present invention will be described below based on embodiments.

First Embodiment

A first embodiment is an example in which combustion system according tothe present invention is applied to a pulverized coal-burning boilerplant, the combustion system being configured to be switchable betweenan air combustion mode in which air is used as carrier gas for solidfuel, first combustion gas and second combustion gas, and anoxygen-based combustion mode in which mixed gas is used as the carriergas, first combustion gas and second combustion gas, where the mixed gasis made by mixing oxygen-rich gas with flue gas produced when the solidfuel is burned. As shown in FIG. 1, an oxygen-based combustion boiler 1adapted to burn solid fuel such as pulverized coal with oxygen-richcombustion gas is installed in the boiler plant according to the firstembodiment. The flue gas discharged from the boiler 1 is introduced intoa denitrification device 3 adapted to remove nitrogen oxides from theflue gas. The denitrification device 3 includes an adding device adaptedto add a reducing agent such as ammonia to the flue gas as well as adenitrification catalyst layer. The denitrification device 3 is designedto render the flue gas harmless by reducing nitrogen oxides in the fluegas to nitrogen. The flue gas discharged from the denitrification device3 is cooled by a heat exchanger 5 and led to a dust collector 7 which isdesigned to collect and remove fly ashes from the flue gas. The flue gasdischarged from the dust collector 7 is introduced into a desulfurizer13 via an induced draft fan 9 and damper 11. The desulfurizer 13 isdesigned to remove sulfur oxides from the flue gas by bringing anabsorbing solution such as a limestone slurry into contact with the fluegas. The flue gas discharged from the desulfurizer 13 is introduced intoa CO₂ recovery device 15. The CO₂ recovery device 15 is designed toseparate CO₂ from the flue gas and store the CO₂ in a CO₂ storagefacility (not shown). A known device can be used as the CO₂ recoverydevice 15, where examples of the known device includes a device whichseparates and recovers CO₂ by compressing the flue gas and liquefyingthe CO₂ or a device which separates and recovers CO₂ by causing the CO₂to be absorbed in an absorbing solution. Other flue gas componentsremaining after the CO₂ is separated by the CO₂ recovery device 15 aredischarged to the atmosphere through a chimney 17.

Next, a burner 19 installed in the boiler 1 according to the firstembodiment will be described. As shown in FIG. 2, the burner 19 isinstalled in a burner throat portion 23 on a side wall of the boiler 1,where the burner throat portion 23 is equipped with water-cooled tubes21. The burner 19 includes a fuel nozzle 25 adapted to burn thepulverized coal supplied in accompaniment with carrier gas, firstcombustion gas nozzles 27 adapted to supply oxygen-rich first combustiongas (hereinafter referred to as additional combustion gas 26) into thefuel nozzle 25, and a second combustion gas nozzle 29 placed on an outerside of the fuel nozzle 25 and adapted to supply oxygen-rich secondarycombustion gas 28 and tertiary combustion gas 30. Acombustion-supporting oil gun 31 is installed on an axial center of thefuel nozzle 25. Plural through-holes are provided in a tube wall of thefuel nozzle 25 to allow insertion of the combustion gas nozzles 27. Thecombustion gas nozzles 27 are inserted in the respective through-holes.The combustion gas nozzles 27 are placed on an inner circumferentialsurface of the fuel nozzle 25, extending along an ejection direction ofa pulverized-coal jet 33 sent by the carrier gas. A nozzle hole 35adapted to eject the additional combustion gas 26 is formed in a tubewall of each combustion gas nozzle 27. The nozzle hole 35 is formed suchthat the additional combustion gas 26 will intersect substantially atright angles with the pulverized-coal jet 33. Also, the nozzle hole 35is designed to supply the additional combustion gas 26 to a firstcombustion zone on a tip side of the fuel nozzle 25. A flow regulatingvalve 37 adapted to control flow rate of the additional combustion gas26 is installed in the combustion gas nozzle 27.

The combustion gas nozzle 29 is formed by a double wall tube concentricwith the fuel nozzle 25. The combustion gas nozzle 29 is designed toeject the secondary combustion gas 28 from an inner tube 39 and ejectthe tertiary combustion gas 30 from an outer tube 41 by branching thecombustion gas supplied to a wind box 24 from a combustion gas line(described later). The inner tube 39 and outer tube 41 are expanded indiameter on the tip side. The inner tube 39 and outer tube 41 areprovided with swirlers 43 and 45 adapted to impart swirling forces tothe secondary combustion gas 28 and tertiary combustion gas 30,respectively. Consequently, the secondary combustion gas 28 and tertiarycombustion gas 30 are ejected through the combustion gas nozzle 29 in aswirling motion while spreading radially outward. This produces a flow(hereinafter referred to as a circulating flow 47) opposite to thepulverized-coal jet 33, between the pulverized-coal jet 33 from the fuelnozzle 25 and swirling flows of the secondary combustion gas 28 andtertiary combustion gas 30. High-temperature gas produced by thecombustion of fuel flows into the circulating flow 47 from downstreamand stagnates there. The pulverized coal in the pulverized-coal jet 33is mixed with the high-temperature gas at an outlet of the burner 19 andraised in temperature by radiant heat from the boiler 1, resulting inignition.

Now, supply systems of carrier gas and combustion gas, which are afeature of the first embodiment, will be described. A branch line 51 isconnected to the flue on an outlet side of the dust collector 7. Aninduced draft fan 53 and damper 55 are installed on the branch line 51to allow a predetermined amount to be branched off from the flue gasfrom which fly ashes have been removed by the dust collector 7. The fluegas branched by the branch line 51 is further branched to a combustiongas line 57 and carrier gas line 59 and vented by a forced draft fan 60and ventilator 62. A draft force of the forced draft fan 60 is designedto be adjusted by adjusting the opening of a damper 61 according to anamount of flue gas to be branched.

A flowmeter 63 adapted to measure flow rate of the flue gas and a mixingdevice 65 adapted to mix the flue gas and oxygen-rich gas are installedon the combustion gas line 57. The mixing device 65 is connected with anoxygen generator 71 via a pipe on which a flowmeter 67 and flowregulating valve 69 are installed. The oxygen generator 71 is, forexample, a known oxygen generator based on a cryogenic separationprocess or the like and is designed to be able to separate oxygen fromair, thereby generate oxygen-rich gas with an oxygen concentration of,for example, 90%, and supply the generated oxygen-rich gas to the mixingdevice 65. The combustion gas line 57 is connected to the wind box 24 ofthe burner 19 via the heat exchanger 5 and is designed to supply thewind box 24 with the combustion gas generated by mixing the flue gas andoxygen-rich gas and heated by the heat exchanger 5.

A flowmeter 73 adapted to measure the flow rate of the flue gas and amixing device 75 adapted to mix the flue gas and oxygen-rich gas areinstalled on the carrier gas line 59. The mixing device 75 is connectedwith the oxygen generator 71 via a pipe on which a flowmeter 77 and flowregulating valve 79 are installed. The heat exchanger 5 and a crusher 81are installed midway along a flow channel of the carrier gas line 59,where the crusher 81 is adapted to crush coal. The crusher 81 issupplied with the carrier gas generated by the mixing device 75 bymixing the flue gas and oxygen-rich gas and is designed to supply thefuel nozzle 25 with pulverized coal from the crusher 81 in accompanimentwith the carrier gas. The carrier gas supplied to the crusher 81 isheated by the heat exchanger 5 through heat exchange with the flue gasand is designed to dry the pulverized coal in the crusher 81.Consequently, the carrier gas line 59 makes up a fuel supply deviceadapted to supply the pulverized coal to the boiler 1. The flow rate ofthe carrier gas supplied to the crusher 81 through the carrier gas line59 is configured to be controllable by a controller (described later).

Characteristic operation of the first embodiment configured in this waywill be described. The boiler plant according to the first embodiment isequipped with a controller (not shown) adapted to set oxygenconcentration in the carrier gas and control flow rate of the carriergas according to a supplied amount and type of pulverized coal, controla supplied amount and oxygen concentration of the additional combustiongas 26 based on the oxygen concentration and flow rate of the carriergas and a first set amount of oxygen needed for combustion of thepulverized coal, and control the secondary combustion gas 28 and controla supplied amount of the secondary combustion gas based on an amount ofoxygen in the carrier gas, an amount of oxygen in the additionalcombustion gas 26, and a second set amount of oxygen needed forcombustion of the pulverized coal.

If, for example, the load of the boiler plant increases, increasing thesupplied amount of pulverized coal, the flow rate of the carrier gas isincreased by increasing the amount of the flue gas running through thecarrier gas line 59. The oxygen concentration in the carrier gas is setto such a level that the coal will not ignite spontaneously duringtransport, based on the type of coal including, for example, the coalrank of the coal and difference in amount of volatile matter. On theother hand, the flue gas contains oxygen remaining after combustion ofthe pulverized coal, and thus the oxygen concentration in the flue gasis determined based on operational status and the like of the plant.Then, a difference between a set oxygen concentration of the carrier gasand oxygen concentration in the flue gas is determined, and oxygen-richgas is mixed in the flue gas so as to eliminate the difference, therebygenerating the carrier gas. That is, the necessary amount of oxygen-richgas is determined based on the flow rate of the flue gas measured by theflowmeter 73 and the opening of the flow regulating valve 79 is adjustedsuch that the necessary amount of oxygen-rich gas will be supplied tothe mixing device 75, based on the flow rate of oxygen-rich gas measuredby the flowmeter 77. Consequently, the amount of oxygen-rich gassupplied to the mixing device 75 is adjusted and the oxygenconcentration in the carrier gas is controlled to match the set oxygenconcentration.

On the other hand, the amount of oxygen to be supplied to the firstcombustion zone is determined from the flow rate and oxygenconcentration of the carrier gas, a difference between the determinedamount of oxygen and the set amount of oxygen which will enablereductive combustion (low-NOx combustion) in the first combustion zoneis determined, and the oxygen concentration and supplied amount of theadditional combustion gas 26 is controlled such that the amount ofoxygen corresponding to the difference will be supplied to the firstcombustion zone. That is, the necessary amount of oxygen-rich gas isdetermined based on the flow rate of the flue gas measured by theflowmeter 63 and the opening of the flow regulating valve 69 is adjustedsuch that the necessary amount of oxygen-rich gas will be supplied tothe mixing device 65, based on the flow rate of oxygen-rich gas measuredby the flowmeter 67. Consequently, the amount of oxygen-rich gassupplied to the mixing device 65 is adjusted and the oxygenconcentration in the additional combustion gas 26 is adjusted. Also, thesupplied amount of the additional combustion gas 26 is controlled byadjusting the opening of the flow regulating valve 37 in the combustiongas nozzle 27.

Now, coordinated control between the oxygen concentration and flow rateof the carrier gas and the oxygen concentration and flow rate of theadditional combustion gas 26 will be illustrated by example. When anexcess oxygen ratio of the boiler 1 is varied from 10 to 40% understeady-state load, the oxygen concentration in the flue gas dischargedfrom the boiler 1 varies from 2 to 8%. Therefore, once the excess oxygenratio of the boiler 1 is determined, the oxygen concentration in theflue gas is determined on its own. Regarding the oxygen-rich gas,although its purity varies with the manufacturing process of oxygen gas,the concentration will seldom change during operation. For example, whenthe boiler 1 is operated at an excess oxygen ratio of 20%, the oxygenconcentration in the flue gas discharged from the boiler 1 will be 5%.If the oxygen concentration of the oxygen-rich gas generated by theoxygen generator 71 is 90%, in order to provide an oxygen concentrationof 10% in the carrier gas line 59, a ratio between the flow rate of theflue gas measured by the flowmeter 73 and the flow rate of oxygen-richgas measured by the flowmeter 77 can be controlled to be equal to 16:1.

Incidentally, an amount of oxygen found by adding an excess amount to atheoretical amount of oxygen needed for combustion of the pulverizedcoal has been set in advance, and a difference between the set amount ofoxygen and the amount of oxygen supplied by the carrier gas andadditional combustion gas 26 is determined. Then, the oxygenconcentrations and supplied amounts of the secondary combustion gas 28and tertiary combustion gas 30 is controlled so as to eliminate thedifference, forming an oxidizing flame around the reducing flamegenerated in the first combustion zone and thereby enabling stablecombustion of the pulverized coal.

Consequently, even if the oxygen concentration in the carrier gas is setlow to prevent spontaneous ignition of the solid fuel during transport,the oxygen deficiency is made up by the additional combustion gas 26.This makes it possible to increase the oxygen concentration in the firstcombustion zone on an outlet side of the fuel nozzle 25 and therebyimprove ignition performance. Accordingly, an amount of oxygen in thefirst combustion zone in the fuel nozzle 25 is set at a level necessaryto maintain the reducing flame. Then, the amount of oxygen needed toburn an unburnt portion is determined from the difference between theamount of oxygen found by adding the excess amount to the theoreticalamount of oxygen needed for combustion of the pulverized coal and theamount of oxygen supplied by the carrier gas and additional combustiongas 26. Since the required amount of oxygen thus determined can besupplemented by the secondary combustion gas 28 and tertiary combustiongas 30, the pulverized coal can be burned stably.

Also, by performing control so as to equalize the oxygen concentrationof the additional combustion gas 26 with the oxygen concentration of thesecondary combustion gas 28 and tertiary combustion gas 30, the firstembodiment can simplify the apparatus and operation. For example, ifdiameter of the burner 19 is not large, control can be performed so asto equalize the oxygen concentration of the additional combustion gas 26with the oxygen concentration of the secondary combustion gas 28 andtertiary combustion gas 30. This makes it possible to control the flowrate of the additional combustion gas 26 and the flow rates of thesecondary combustion gas 28 and tertiary combustion gas 30 using theflow regulating valve 37 and thereby supply the necessary amount ofoxygen in each of the combustion gases. Incidentally, if the diameter ofthe burner 19 is large, separate gas lines can be installed so that theoxygen concentration of the additional combustion gas 26 and the oxygenconcentration of the secondary combustion gas 28 and tertiary combustiongas 30 can be controlled separately.

Also, according to the first embodiment, the combustion gas suppliedfrom the combustion gas line 57 to the wind box 24 is divided into theadditional combustion gas 26 and the secondary combustion gas 28 andtertiary combustion gas 30 using the flow regulating valve 37. However,separate gas lines may be installed to control the flow rate of theadditional combustion gas 26 and the flow rate of the secondarycombustion gas 28 and tertiary combustion gas 30 independently.

Also, during a start-up operation, a plant with a combustion system suchas the one according to the first embodiment will run short of flue gas,and thus operate in the air combustion mode in which air is used ascombustion gas for pulverized coal. Then, with increases in the fluegas, the plant is switched to the oxygen-based combustion mode in whichmixture of the flue gas and oxygen-rich gas is used as the combustiongas. In so doing, when the flue gas increases during the start-upoperation, the flue gas is substituted for the air. In this case, whilethe oxygen concentration in the carrier gas is high due to the admixtureof air, control is performed so as to keep down the amount ofoxygen-rich gas supplied to the carrier gas and set the flow rate oroxygen concentration of the additional combustion gas 26 to a low level.In this way, plant is operated by switching between the air combustionmode and oxygen-based combustion mode.

Also, under high load at which the flow rate of the carrier gas is high,flames tend to leap up, and thus control is performed so as not todecrease the oxygen concentration in the carrier gas too much.

Incidentally, with the oxygen-based combustion system, since thecombustion gas is produced by diluting oxygen-rich gas with flue gas,the CO₂ concentration in the combustion gas reaches, for example, 60 to90%, flame spread speed is drawn far low in comparison with the air,resulting a delay of an ignition. Also, an amount of heat absorption ina furnace of the boiler 1 can get lower than with an air combustionmethod. Thus, preferably the oxygen concentration in the carrier gas isset to 21% or below and the oxygen concentration of the additionalcombustion gas 26 as well as the oxygen concentration of the secondarycombustion gas 28 and tertiary combustion gas 30 are controlled to be,for example, 25 to 35%. That is, preferably the oxygen concentration inthe combustion zone is controlled to be higher than during aircombustion. Incidentally, if the oxygen concentration is increased toomuch, for example, if the oxygen concentration exceeds 50%, componentsmight be damaged by ignition/explosion of the fuel upstream of theburner 19 or abnormally high temperature of the burner 19. Therefore,preferably the oxygen concentration is controlled so as not to exceed35%. Incidentally, a lower limit of the oxygen concentration in thecarrier gas can be set appropriately according to the type of coal. Forexample, depending on burner structure and gas flow velocity, bituminouscoal has low flame stability when the oxygen concentration is less than10% and becomes difficult to ignite when the oxygen concentration fallsto about 5%.

The flue gas can be branched by connecting the branch line 51 to theflue on an outlet side of the desulfurizer 13 rather than on the outletside of the dust collector 7. That is, preferably the carrier gas andcombustion gas are generated using flue gas from which at least flyashes have been removed.

Also, to measure the oxygen concentrations in the combustion gas line 57and the carrier gas line 59, an analyzer adapted to analyze oxygenconcentrations in piping may be installed in addition to determinationby means of flowmeters. In that case, since usual analyzers takemeasurements on a dry basis, it is desirable to make evaluations on awet basis by installing a moisture meter jointly. Desirably the analyzeris installed downstream of the heat exchanger 5. The oxygenconcentration of the additional combustion gas 26 as well as the oxygenconcentration of the secondary combustion gas 28 and tertiary combustiongas 30 can be measured by installing an oxygen concentration meter inthe wind box 24. Desirably the oxygen concentration in the carrier gasis measured in the fuel nozzle 25, but since it is conceivable that ananalyte-gas sampling hole may be clogged with fuel, the oxygenconcentration may be measured between the heat exchanger 5 and crusher81 when a purge function is not provided.

Also, when a combustion system such as described above is constructed byremodeling an existing air combustion boiler plant and/or addingfacilities to the existing air combustion boiler plant, it is desirableto avoid major remodeling of the heat exchanger and so on in the boiler1 from the economic standpoint.

Second Embodiment

A block diagram of a boiler plant according to a second embodiment isshown in FIG. 3. The second embodiment differs from the first embodimentin that the flue gas is used directly as carrier gas. That is, themixing device 75 and flowmeter 73 on the carrier gas line 59 as well asthe flowmeter 77 and flow regulating valve 79 for oxygen-rich gas areomitted, and the flue gas is used as carrier gas without being mixedwith oxygen-rich gas. Other components are the same as the firstembodiment, and thus denoted by the same reference numerals as thecorresponding components in the first embodiment and description thereofwill be omitted.

For example, even if the carrier gas supplied to the fuel nozzle 25 hasa low oxygen concentration, coal with good combustion characteristics,such as lignite, can achieve stable ignition as long as the additionalcombustion gas 26 has a high oxygen concentration. Also, when coal withgood combustion characteristics is used, it is advisable not to mixoxygen because the fuel might ignite spontaneously or explode in somemidpoint in piping if oxygen is mixed in the carrier gas line 59. Thus,according to the second embodiment, the flue gas is used directly ascarrier gas without adding oxygen-rich gas to the flue gas. In thiscase, the oxygen concentration in the flue gas on the outlet side of theboiler 1 determined based on the operational status and the like of theplant is used as the oxygen concentration in the carrier gas. This makesit possible to omit control over the oxygen concentration in the carriergas and thereby reduce facility and operating costs.

Third Embodiment

A burner suitable for the boiler plant according to the first or secondembodiment is shown in FIG. 4 as a third embodiment. As shown in FIG. 4,a burner 90 according to the third embodiment differs from the burner 19according to the first embodiment in that a venturi 91 shaped tosmoothly contract and expand a sectional area of the flow channel froman outer peripheral side is formed on the upstream side of the fuelnozzle 25, that a concentrator 93 shaped to smoothly contract and expanda sectional area of the flow channel of the fuel nozzle 25 from insideis installed on the downstream side of the venturi 91, and that theadditional combustion gas 26 is supplied to the downstream side of theconcentrator 93. Furthermore, a barrier called a flame holder 95 isinstalled on an exterior side of a tip portion of the fuel nozzle 25,i.e., on an outlet side of the furnace of the boiler 1. Other componentsare the same as the first embodiment, and thus denoted by the samereference numerals as the corresponding components in the firstembodiment and description thereof will be omitted.

The flame holder 95 acts as a barrier to the pulverized-coal jet 33 andflow of secondary combustion gas 28 ejected from the fuel nozzle 25.Therefore, since the pressure on the downstream side of the flame holder95 can be reduced, the circulating flow 47 grows large. Consequently,high-temperature gas flows in from the furnace of the boiler 1 moreintensely than in the first embodiment. On the other hand, thepulverized coal is condensed on an outer peripheral side of the fuelnozzle 25 by the venturi 91 and concentrator 93. This improvesmixability with the additional combustion gas 26 supplied to the outerperipheral side of the fuel nozzle 25. Consequently, the pulverized coalis condensed and the pulverized-coal jet 33 with a high oxygenconcentration is produced near the flame holder 95 and flows into thecirculating flow 47, making the ignition of pulverized coal faster thanin the first embodiment. This allows the oxygen concentration in thecarrier gas line to be made lower than in the first embodiment.

Fourth Embodiment

A burner suitable for the boiler plant according to the first or secondembodiment is shown in FIG. 5 as a fourth embodiment. A burner 97according to the fourth embodiment differs from the burner 90 accordingto the third embodiment in that a distributor 99 is placed to partitionthe flow channel in the fuel nozzle 25 on the downstream side of theconcentrator 93. Other components are the same as the third embodiment,and thus denoted by the same reference numerals as the correspondingcomponents in the third embodiment and description thereof will beomitted.

The distributor 99 is a ring-shaped member formed along an axialdirection of the fuel nozzle 25 and is placed concentrically with thefuel nozzle 25. When viewed in a direction perpendicular to an axis ofthe burner 97, the nozzle holes 35 of the combustion gas nozzles 27 areformed at locations overlapping the distributor 99. The partitioning ofthe flow channel by the distributor 99 keeps the pulverized coal,carrier gas, and additional combustion gas 26 from mixing with oneanother. This makes it possible to prevent dispersion of the pulverizedcoal due to collisions among pulverized coal particles as well asdiffusion of the additional combustion gas 26 to near a central axis.Consequently, gas with a low oxygen concentration and gas with a highoxygen concentration can be separated at an outlet of the burner 97 withthe former directed onto the central axis and the latter directed towardan outer circumference. This increases the oxygen concentration in thecarrier gas around the pulverized coal near the flame holder 95,enabling stable ignition. Incidentally, with the fourth embodiment, thecombination of the concentrator 93 and distributor 99 allows thepulverized coal to be physically gathered in a circumferential directionof the fuel nozzle 25. Also, the additional combustion gas 26 with ahigh oxygen concentration ejected from the combustion gas nozzles 27 isthrown in the circumferential direction of the fuel nozzle 25, passingbetween the distributor 99 and an inner wall of the fuel nozzle 25. Thismakes it possible to facilitate mixing of the additional combustion gas26 and pulverized coal, making the ignition of pulverized coal near theflame holder 95 faster than with the burner 90 according to the thirdembodiment and reducing the oxygen concentration in the carrier gas.

Fifth Embodiment

A burner suitable for the boiler plant according to the first or secondembodiment is shown in FIG. 6 as a fifth embodiment. A burner 101according to the fifth embodiment differs from the burner 97 accordingto the fourth embodiment in that combustion gas nozzles 105 are formedsuch that the additional combustion gas 26 will be ejected vertically tothe pulverized-coal jet 33 from the circumferential direction of thefuel nozzle 25 and that the venturi 91 and concentrator 93 are omitted.Other components are the same as the fourth embodiment, and thus denotedby the same reference numerals as the corresponding components in thefourth embodiment and description thereof will be omitted.

Plural combustion gas nozzles 105 adapted to eject the additionalcombustion gas 26 onto the central axis of the burner are placed on anexternal wall of the fuel nozzle 25. The additional combustion gas 26ejected substantially vertically to the pulverized-coal jet 33 collideswith the distributor 99 and thereby forms a high oxygen concentrationzone in outer peripheral part of the fuel nozzle 25 without spreadingtoward a center of the burner 101. This makes it possible to achievestable ignition at the burner outlet.

Sixth Embodiment

A burner suitable for the boiler plant according to the first or secondembodiment is shown in FIG. 7 as a sixth embodiment. A burner 111according to the sixth embodiment differs from the burner 101 accordingto the fifth embodiment in that the venturi 91 and concentrator 93 areinstalled upstream of the distributor 99. Regarding other components,the other components are the same as the fifth embodiment, and thusdenoted by the same reference numerals as the corresponding componentsin the fifth embodiment and description thereof will be omitted.

This makes it possible to condense pulverized coal in an outercircumferential direction of the fuel nozzle 25 and form a zone with ahigh oxygen concentration using the concentrator 93 and distributor 99.Also, the flame holder 95 forms a large backflow zone, causinghigh-temperature gas to flow in from the furnace and therebyfacilitating ignition and combustion of pulverized coal near the flameholder 95 on the outer circumference of the fuel nozzle 25 at the burneroutlet.

Example

Now, experimental results on stability evaluations of flames produced bythe burners according to the first to sixth embodiments described abovewill be described as an example. The oxygen-based combustion system, inwhich combustion gas has a high carbon dioxide concentration asdescribed above, is poorer in flame spread than air combustion.Consequently, if the oxygen-based combustion system is configured toadjust the oxygen concentration in the carrier gas to be lower than theoxygen concentration (21%) in air to prevent spontaneous ignition ofpulverized coal, the flame stability is compromised. Thus, in the firstto sixth embodiments, combustion gas nozzles are provided to supplyadditional combustion gas to the burners. Experimental results whichevaluate consequent flame stability on the burners are shown in Table 1.Incidentally, a burner not provided with a combustion gas nozzle and notsupplied with additional combustion gas is shown in Table 1 as acomparative example.

TABLE 1 Oxygen Concentration in Carrier Gas 24% 21% 15% 10% 5% Coal TypeA B C A B C A B C A B C A B C First Embodiment ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ X Δ ◯ XX Δ Second Embodiment — — — — — — — — — — — — X X Δ Third Embodiment ◯ ◯◯ ◯ ◯ ◯ ◯ ◯ ◯ X ◯ ◯ X X Δ Fourth Embodiment ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ X Δ◯ Fifth Embodiment ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ X X ◯ X X Δ Sixth Embodiment ◯ ◯ ◯◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ X X Δ Burner of ◯ ◯ ◯ X ◯ ◯ X X ◯ X X X X X XComparative Example

In the present experiment, the average oxygen concentration of all thegases supplied through the entire burner—i.e., the carrier gas,additional combustion gas 26, secondary combustion gas 28, and tertiarycombustion gas 30 taken together—was 27%. Flame stability was visuallyevaluated at oxygen concentrations of 24%, 21%, 15%, 10%, and 5% in thecarrier gas. Specifically, images of high-temperature flames werevisually checked, and a rating of stable ◯ was given when a flame wasformed by the entire area of the burner base (furnace-side openingportion (aperture) of the burner), a rating of unstable x was given whenwhite portion of the flame existed only in part of the burner base, anda rating of medium Δ of stable and unstable was given when the flamecondition was somewhere in between. Also, the flame stability ofmultiple different coal types (A: bituminous coal; B: subbituminouscoal; and C: lignite) was evaluated. Regarding the second embodiment, inwhich the carrier gas was made up only of flue gas without admixture ofoxygen-rich gas, the flame stability was evaluated only at an oxygenconcentration of 5%, which was the oxygen concentration in the flue gas.

As shown in Table 1, with the burner according to the comparativeexample, all the coal types produced stable flames when the oxygenconcentration in the carrier gas was 24%, which is higher than theoxygen concentration in air. However, when the oxygen concentration waslowered to 21%, equivalent to that in air, bituminous coal produced anunstable flame. Then, as the oxygen concentration was lowered to 10%,all the coal types produced unstable flames.

In contrast, with the burners according to the first and third to sixthembodiments, all the coal types produced stable flames at oxygenconcentrations from 24% which is higher than in air to 15% which islower than in air. At an oxygen concentration of 10%, combinations ofsome burners and some coal types produced unstable flames, but ligniteproduced stable flames with all the burners. Furthermore, as the oxygenconcentration was lowered, the flames became unstable, making ignitiondifficult in a growing number of cases. However, when lignite was burnedby the burner according to the fourth embodiment, the flame was stableeven at an oxygen concentration of 5%.

For example, the burner according to the fourth embodiment enabledstable combustion without causing the flame to leap up from the burnerbase (to separate from the aperture) when the oxygen concentration inthe carrier gas was 21% which was approximately equivalent to that inair, and even 10% which was less than half that in air. On the otherhand, with the burner according to the comparative example, when theoxygen concentration in the carrier gas was 21% which was approximatelyequivalent to that in air, the flame was unstable, leaping up from theburner base (separating widely from the aperture). Incidentally,preferably the oxygen concentration in the carrier gas is adjusted to belower than the oxygen concentration (approximately 21%) in air toprevent spontaneous ignition of pulverized coal, and preferably theadditional combustion gas is adjusted to be equal to or higher in oxygenconcentration than air.

In this way, the lower limit of oxygen concentration in the carrier gasfor a stable flame varies with the conditions, including the coal type,which affect combustion characteristics but the burners according to thefirst and third to sixth embodiments, when used by supplying theadditional combustion gas, can stabilize the flames on the burners. Thatis, even if the oxygen concentration in the carrier gas is lower thanthe oxygen concentration (approximately 21%) in air, a stable flame canbe produced, making it possible to prevent spontaneous ignition ofpulverized coal reliably by decreasing the oxygen concentration in thecarrier gas. In other words, by adjusting the oxygen concentration inthe carrier gas to be lower than the average oxygen concentration of thecarrier gas, additional combustion gas, secondary combustion gas, andtertiary combustion gas taken together and by adjusting the oxygenconcentration in additional combustion gas to be higher than the averageoxygen concentration of the carrier gas, additional combustion gas,secondary combustion gas, and tertiary combustion gas taken together, itis possible to implement a combustion system which can produce a stableflame.

REFERENCE SIGNS LIST

-   1 Boiler-   7 Dust collector-   15 CO₂ recovery device-   19 Burner-   25 Fuel nozzle-   26 Additional combustion gas-   27 Combustion gas nozzle-   28 Secondary combustion gas-   29 Combustion gas nozzle-   30 Tertiary combustion gas-   57 Combustion gas line-   59 Carrier gas line

1. A combustion system wherein: a burner adapted to burn solid fuelcomprises a fuel nozzle adapted to burn solid fuel supplied inaccompaniment with carrier gas, a first combustion gas nozzle adapted tosupply first combustion gas into the fuel nozzle, a second combustiongas nozzle placed on an outer side of the fuel nozzle and adapted tosupply second combustion gas; the combustion system is configured to beswitchable between an air combustion mode in which air is used as thecarrier gas, the first combustion gas and the second combustion gas, andan oxygen-based combustion mode in which mixed gas is used as thecarrier gas, the first combustion gas and the second combustion gas,where the mixed gas is made by mixing oxygen-rich gas with flue gasproduced when the solid fuel is burned; respective oxygen concentrationsand supplied amounts of the carrier gas, the first combustion gas, andthe second combustion gas are each configured to be adjustable; and inthe oxygen-based combustion mode, the oxygen concentration in thecarrier gas is adjusted to be lower than an average oxygen concentrationof the carrier gas, the first combustion gas and the second combustiongas taken together, and the oxygen concentration in the first combustiongas is adjusted to be equal to or higher than the average oxygenconcentration of the carrier gas, the first combustion gas and thesecond combustion gas taken together.
 2. The combustion system accordingto claim 1, wherein: the fuel nozzle includes a distributor adapted todivide a flow channel in the fuel nozzle; and an outlet of the firstcombustion gas nozzle is formed at a location overlapping thedistributor when viewed in a direction perpendicular to a burner axis.3. The combustion system according to claim 1, wherein: the fuel nozzleincludes a venturi and a concentrator located in a flow channel upstreamof a position at which the first combustion gas nozzle is connected, theventuri being adapted to smoothly contract and expand a sectional areaof the flow channel of the fuel nozzle from an outer peripheral sidewhile the concentrator being adapted to smoothly expand the sectionalarea of the flow channel downstream of the venturi from inside; and aflame holder is installed at a tip of a partition wall partitioning thefuel nozzle and the second combustion gas nozzle, the flame holder beingadapted to block flow of the solid fuel and carrier gas ejected throughthe fuel nozzle and flow of the first combustion gas ejected through thefirst combustion gas nozzle.
 4. A combustion system comprising: anoxygen-based combustion boiler adapted to burn solid fuel by dilutingoxygen-rich gas with flue gas; a crusher adapted to crush the solidfuel; a fuel supply device adapted to supply the crushed solid fuel to aburner of the boiler in accompaniment with carrier gas; a dust collectoradapted to collect fly ashes in flue gas discharged from the boiler; anda carbon dioxide recovery unit adapted to separate and recover carbondioxide from the flue gas discharged from the dust collector; the burnerin turn comprising a fuel nozzle adapted to burn the solid fuel suppliedin accompaniment with the carrier gas, a first combustion gas nozzleadapted to supply first combustion gas into the fuel nozzle, and asecond combustion gas nozzle placed on an outer side of the fuel nozzleand adapted to supply second combustion gas, wherein the combustionsystem further comprises a carrier gas line adapted to generate thecarrier gas from the flue gas branching off from a downstream side ofthe dust collector, a combustion gas line adapted to generate combustiongas by adding oxygen-rich gas to the flue gas branching off from thedownstream side of the dust collector, and a controller adapted to setoxygen concentration in the carrier gas and control flow rate of thecarrier gas according to a supplied amount and type of the solid fuel,control a supplied amount and oxygen concentration of the firstcombustion gas based on the oxygen concentration and flow rate of thecarrier gas and a set amount of oxygen needed for combustion of thesolid fuel, and control a supplied amount of the second combustion gasbased on an amount of oxygen in the carrier gas, an amount of oxygen inthe first combustion gas, and a second set amount of oxygen needed forcombustion of the solid fuel.
 5. The combustion system according toclaim 4, wherein an adding device adapted to add oxygen-rich gas to theflue gas based on the oxygen concentration in the carrier gas set by thecontroller is installed on the carrier gas line.
 6. The combustionsystem according to claim 4, wherein the controller sets the oxygenconcentration in the carrier gas to 21% or below and controls the oxygenconcentrations in the first and second combustion gases to be 25 to 35%.7. A combustion system wherein: a burner adapted to burn solid fuelcomprises a fuel nozzle adapted to burn solid fuel supplied inaccompaniment with carrier gas, a first combustion gas nozzle adapted tosupply first combustion gas into the fuel nozzle, a second combustiongas nozzle placed on an outer side of the fuel nozzle and adapted tosupply second combustion gas; the combustion system is configured to beswitchable between an air combustion mode in which air is used as thecarrier gas, the first combustion gas, and the second combustion gas andan oxygen-based combustion mode in which the fuel gas produced bycombustion of the solid fuel is used as the carrier gas while mixed gasis used as the first combustion gas, and the second combustion gas,where the mixed gas is made by mixing oxygen-rich gas with flue gasproduced when the solid fuel is burned; and in the oxygen-basedcombustion mode, the oxygen concentration in the carrier gas is set tobe lower than an average oxygen concentration of the carrier gas, thefirst combustion gas, and the second combustion gas taken together andthe oxygen concentration in the first combustion gas is adjusted to beequal to or higher than the average oxygen concentration of the carriergas, the first combustion gas, and the second combustion gas takentogether.
 8. An operating method for a combustion system which comprisesan oxygen-based combustion boiler adapted to burn solid fuel by dilutingoxygen-rich gas with flue gas, a crusher adapted to crush the solidfuel, a fuel supply device adapted to supply the crushed solid fuel to aburner of the boiler in accompaniment with carrier gas, a dust collectoradapted to collect fly ashes in flue gas discharged from the boiler, anda carbon dioxide recovery unit adapted to separate and recover carbondioxide from the flue gas discharged from the dust collector, the burnerin turn comprising a fuel nozzle adapted to burn the solid fuel suppliedin accompaniment with the carrier gas, a first combustion gas nozzleadapted to supply first combustion gas into the fuel nozzle, and asecond combustion gas nozzle placed on an outer side of the fuel nozzleand adapted to supply second combustion gas, the operating methodcomprising: generating the carrier gas by selectively adding oxygen-richgas to the flue gas branching off from the downstream side of the dustcollector; generating the first combustion gas and the second combustiongas by adding oxygen-rich gas to the flue gas branching off from thedownstream side of the dust collector; and setting oxygen concentrationin the carrier gas and controlling flow rate of the carrier gasaccording to a supplied amount and type of the solid fuel, controlling asupplied amount and oxygen concentration of the first combustion gasbased on the oxygen concentration and flow rate of the carrier gas and afirst set amount of oxygen needed for combustion of the solid fuel, andcontrolling a supplied amount of the second combustion gas based on anamount of oxygen in the carrier gas, an amount of oxygen in the firstcombustion gas, and a second set amount of oxygen needed for combustionof the solid fuel.
 9. The operating method for a combustion systemaccording to claim 8, wherein during a start-up operation, the carriergas is generated by making up deficiency of the flue gas branching offfrom the downstream side of the dust collector, using air.
 10. Theoperating method for a combustion system according to claim 8, whereinthe oxygen concentration in the carrier gas is set to 21% or below andthe oxygen concentrations in the first and second combustion gases arecontrolled to be 25 to 35%.
 11. The operating method for a combustionsystem according to claim 9, wherein the oxygen concentration in thecarrier gas is set to 21% or below and the oxygen concentrations in thefirst and second combustion gases are controlled to be 25 to 35%. 12.The combustion system according to claim 2, wherein: the fuel nozzleincludes a venturi and a concentrator located in a flow channel upstreamof a position at which the first combustion gas nozzle is connected, theventuri being adapted to smoothly contract and expand a sectional areaof the flow channel of the fuel nozzle from an outer peripheral sidewhile the concentrator being adapted to smoothly expand the sectionalarea of the flow channel downstream of the venturi from inside; and aflame holder is installed at a tip of a partition wall partitioning thefuel nozzle and the second combustion gas nozzle, the flame holder beingadapted to block flow of the solid fuel and carrier gas ejected throughthe fuel nozzle and flow of the first combustion gas ejected through thefirst combustion gas nozzle.
 13. The combustion system according toclaim 5, wherein the controller sets the oxygen concentration in thecarrier gas to 21% or below and controls the oxygen concentrations inthe first and second combustion gases to be 25 to 35%.